There’s an old riddle about the surgeon’s son. Here’s Douglas Hofstadter’s version of it, from his Metamagical Themas column in the November 1982 edition of Scientific American, as reprinted in his book of the same name.

A father and his son were driving to a ball game when their car stalled on the railroad tracks. In the distance a train whistle blew a warning. Frantically, the father tried to start the engine, but in his panic, he couldn’t turn the key, and the car was hit by the onrushing train. An ambulance sped to the scene and picked them up. On the way to the hospital, the father died. The son was still alive but his condition was very serious, and he needed immediate surgery. The moment they arrived at the hospital, he was wheeled into an emergency operating room, and the surgeon came in, expecting a routine case. However, on seeing the boy, the surgeon blanched and muttered, “I can’t operate on this boy — he’s my son.”

What do you make of this grim riddle? How could it be? Was the surgeon lying or mistaken? No. Did the dead father’s soul somehow get reincarnated in the surgeon’s body? No. Was the surgeon the boy’s true father and the dead man the boy’s adopted father? No. What, then, is the explanation? Think it through until you have figured it out on your own — I insist! You’ll know when you’ve got it, don’t worry.

It is a fun puzzle to pose, because not only do people do the normal squirming before the puzzle is solved, but often they squirm more afterwards. It is also remarkable the elaborate plots people produce before they hit on the simple answer.

A rather less tragic version of this puzzle came to life at a party this weekend.

I was chatting to a group of people, including a doctor who mentioned his mother met a friend of mine years ago at medical school. I said “So, you are the son of a doctor?” and he replied “Actually, I’m the son of two doctors.”

That’s when Jack, a wide-eyed 16-year old, wandered into the conversation, and asked “What? How does that work? Was your father and your grandfather doctors?” There was nervous laughter from the group, which confused poor Jack, so he plunged on. “No, I am serious, I don’t get it.”

When the situation was explained to him, the rest of the group were smirking to each other, daring each other to get stuck into him – but there was no need. Jack was mortified, and spent the rest of the evening shaking his head at himself, saying “I can’t believe I said that!” over and over again. It never stopped (well, to be fair, whenever it paused for a while, someone would slyly reference it again, and off he would go again!)

I figured it out after about ten or twenty seconds, but I don’t see a reason to be embarrassed by not thinking of it. The answer is something that is obviously possible, but statistically uncommon. Your brain tends to have an easier time thinking of things it sees frequently. That’s not strange, and it isn’t a statement about your personality or beliefs.